1. Field of the Invention
The present invention relates to a geared motor, and more particularly to a geared motor that can rapidly and precisely detect the mechanical starting point of an output shaft of a reduction gear during motor startup and at other times.
2. Description of the Related Art
Geared motors are used that are configured to output the rotational output of a motor in the drive section of industrial robots, machine tools, and the like via a reduction gear having high transmission accuracy. Such drive sections require high positioning accuracy. As shown in FIG. 11, a geared motor 101 has a motor main body 102, a reduction gear 103 coaxially connected to a motor shaft 102a of the motor main body 102, and an output shaft 104 coaxially connected to the output side of the reduction gear 103. A wave gear drive, for example, is used as the reduction gear 103.
In a geared motor 101, the rotational angle of the output shaft 104 of the reduction gear 103 must be controlled with high precision for accurate positioning or the like. For this reason, a motor encoder 106 is mounted on the motor shaft 102a, and an origin sensor 107 is mounted on the output shaft 104.
In a signal processing circuit 108, based on the A-, B-, and Z-phase signals obtained from the motor encoder 106, and the origin signal S that generates a single pulse per single rotation and is obtained from the origin sensor 107, commands are sent to the motor driver 109 so that the output shaft 104 achieves a desired rotational angle. The motor driver 109 rotatably drives the motor shaft 102a in accordance with commands thus received.
In the geared motor 101, the rotational angle position of the output shaft 104 is controlled based on the mechanical starting point of the shaft. Therefore, at startup and at other times, the output shaft 104 must be returned to the mechanical starting point (origin position).
Nevertheless, there is a problem in a conventional geared motor in that a considerable amount of time is required in the origin return movements. More specifically, as shown in FIG. 12, in the origin return movements of the output shaft 104, it is necessary that the motor shaft 102a be rotated (first movement) until an origin signal is output from the origin sensor 107 mounted on the output shaft 104, the motor shaft 102a then be rotated in the opposite direction to return the output shaft 104 (second movement) to the rotational angle position at a point just before the origin signal S is output, and the motor shaft 102a be again rotated in the forward direction and stopped (third movement) in the rotational position at which the first Z-phase signal is output following the output of the origin signal.
In these movements, the motor shaft 102a must be rotated by an amount equal to the rotational angle that corresponds to the reduction gear ratio of the reduction gear at most. When the reduction gear ratio is 1:50, for example, the motor shaft 102a must be rotated 50 times, that is, 18,000 degrees (50×360°), and a considerable amount of time is required.